Improve available output

The available command can now:

• differentiate between the toolchains and the package_sets
• show the packages that packages_sets have:

❯ ./mpsd-software-environment.py available dev-23a

MPSD software release dev-23a provides:
Toolchains: 
    foss2021a-cuda-mpi
    foss2021a-mpi
    foss2021a-serial
    foss2022a-cuda-mpi
    foss2022a-mpi
    foss2022a-serial
Package sets:
   global_generic ( anaconda3@2022.10 )  
   global ( octopus@12.1, octopus@12.1 )